This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9703617-2 filed in Sweden on Oct. 3, 1997; the entire content of which is hereby incorporated by reference.
The present invention relates to a down/up-converter and a down/up-conversion method for high/low frequency signals.
Down- and up-conversion is used in, for example, single sideband demodulators and modulators. A common technique (see [1]) for demodulation involves a mixer, which multiplies the incoming signal with two 90xc2x0 phase-shifted sinusoidal signals, and a Hilbert filter for one of the resulting components (in-phase or quadrature component). The output signal from the Hilbert filter is then added or subtracted to or from the other component, depending on whether the lower or upper sideband is desired. Modulation is performed by using the same elements in the reverse order.
A drawback of this technique is that it is very difficult to design a lowpass Hilbert filter both in analog and digital form. Furthermore, in practice it may be difficult to perform the down-conversion to or up-conversion from baseband in one step (for example, cellular telephony systems often operate with carrier frequencies of the order of 1 GHz).
An object of the present invention is to provide an apparatus and method that avoids Hilbert filters.
Another object is to perform the down- or up-conversion in several mixing steps.
These objects are achieved with an apparatus and method in accordance with the appended claims.
Briefly, the present invention is based on the insight that complex filters automatically provide a 90xc2x0 phase-shift between real and imaginary components of a complex signal. This feature, when combined with a second mixer, may be used to eliminate the undesirable Hilbert filter of the prior art. At the same time a two step conversion involving two mixers is obtained.